The benefits of genetics

Genetics is a medical specialty dedicated to genetic disease and heredity research. It is intended for patients of all ages (prenatal to adult) and includes several other medical specialties, including pediatrics, oncology, cardiology and neurology.

We all have 23 pairs of chromosomes (22 pairs called autosomes and a pair of sex chromosomes) that determine our sex (XX in women, and XY in men). Each chromosome contains several genes that provide the necessary instructions for our development and for the functions of our body. All of our genes exist in two copies, one from our mother and one from our father. Some of our genes are non-functional because they contain a change called a mutation that prevents their normal function. The impact of these mutations varies depending on the mode of transmission.

DNA is the alphabet that makes up our genetic code. In some cases, an error in our chromosomes or genes results in dysfunction or malformation, such as:

• Supernumerary chromosome: Trisomy 21 (Down syndrome) or Klinefelter syndrome (47,XXY)
• Missing chromosome: Turner’s syndrome (45,X)
• Missing part of a chromosome: 22q11.2 deletion syndrome (DiGeorge’s syndrome)

• Substitution (change of one “letter” to another)
• Deletion (a portion of the gene is absent)
• Duplication (a portion of the gene is repeated)

Some mutations are called “de novo” or sporadic. That is, the mutation appears for the first time in an affected individual, but is not transmitted by one of the parents. The mutation usually occurs at the time the embryo is conceived, or shortly thereafter. In this situation, other family members are not likely to have the same mutation. However, the person with the disease will be at risk of passing the disease on to his or her children. Certain genetic diseases often appear de novo, such as achondroplasia (small person), neurofibromatosis, Duchenne muscular dystrophy.

Many genetic diseases can appear at birth or later in life. Our chromosomes contain 20,000 to 25,000 DNA segments (genes), which include instructions for producing all the proteins necessary to sustain life. Each of these genes can be altered by a mutation that changes the structure of a protein. Currently, more than 6,000 diseases are associated with a genetic mutation.

We also have a second type of DNA that is specific to mitochondria (cell organelle responsible for providing cell energy). This DNA is transmitted only by the mothers (through the egg). Genetic errors in mitochondrial DNA are also associated with diseases such as Leigh’s syndrome (metabolic disease affecting the nervous system) and Leber’s optical neuropathy (vision loss).

Medical genetics includes several sub-specialties:

• Prenatal, pediatric or adult genetics, which consists of diagnosing rare diseases of suspected genetic origin and counselling families. Prenatal (e.g., at-risk pregnancy, identified abnormalities after amniocentesis) and postnatal (e.g., mental retardation, malformations) consultations are proposed
• Genetic oncology, which focuses on genetic cancer risks and prevention in families (e.g., enhanced follow-up, risk reduction surgery)
• Neurogenetics, which affects muscular and neurological diseases (e.g., dystrophies, Huntington’s disease, Alzheimer’s, some forms of epilepsy)
• Cardiology that focuses on hereditary heart disease (cardiomyopathic hypertrophy, arrhythmias or long QT syndrome)
• Molecular genetics and cytogenetics, which affect laboratory analyses, involve the development of diagnostic tests and the identification of new genetic mutations.

There are thousands of genetic diseases, most often rare (affecting less than one in 2,000 people). Among the most common are:

• Cystic fibrosis (or mucoviscidosis)
• Hemochromatosis (excess iron in the body)
• Trisomy 21 (or Down syndrome)
• Myopathies (genetic diseases affecting the muscles), such as Duchenne muscular dystrophy or Steinert’s disease
• Breast cancers linked to BRCA1 or BRCA2 mutation
• Colon cancer related to a mutation in the MLH1, MSH2 or MSH6 genes
• Hemophilia
• Tay Sachs disease
• Sickle cell anemia
• Thalassemia (red blood cell diseases)
• Huntington’s disease
• Fragile X syndrome

If a member of your family has a genetic disease, your risk of being affected or transmitting the disease is potentially higher.

Genetic testing can help you know, for example, if you or your child are likely to develop certain genetic conditions (transmitted by your genes). These tests are usually conducted when there is a family history of a genetic disease or to determine a genetic cause that explains certain symptoms.

They can benefit individuals, couples and families who have genetic concerns such as:

• Family history of cancer
• Family history of diseases that can be hereditary (e.g., cancer, heart problems, Alzheimer’s disease, Parkinson’s disease)
• Risk of carrying or having a child with a hereditary disease (link to info in Small Guides – Pregnancy)

There are several types of genetic tests, each with its strengths and weaknesses.

As the name suggests, this type of test is performed for the purpose of diagnosing and determining a genetic cause that explains symptoms or a disease. This test usually targets one or more genes known to be associated with the disease in question. The tests are performed in a clinical laboratory that meets established standards and has received the required certifications. Tests are always requested as part of a medical follow-up and the results are given to you by a health professional who ensures understanding and management. The results are part of the patient’s medical record.

Examples include:

• Gene testing for predisposition to hereditary breast cancer: BRCA1 and BRCA2
• Carrier test for cystic fibrosis
• Test to confirm diagnosis of Duchenne muscular dystrophy
• Amniocentesis for trisomy

The term “genetic screening” is used when the test identifies individuals who are more likely to have a genetic disease. However, further testing is required to confirm or eliminate the diagnosis.

Examples:

• Screening for Down syndrome during pregnancy. Screening gives you a higher or lower risk. If the risk is higher, an amniocentesis may be offered to confirm or eliminate the risk.
• Newborn screening. Biochemical analyses can identify a child who may be susceptible to genetic disease, but a diagnostic test is needed to confirm or eliminate this risk.

These tests are similar to diagnostic tests. However, the tests are conducted as part of a research protocol and, according to the study, the results are not necessarily given to you. In addition, some research may take place over a period of months or years. If a genetic research result is given to the participant, the result must be confirmed by a clinical laboratory that complies with established standards and has received the required approvals before considering this result as being diagnostic.

Advances in technology now allow several genes (panels) to be tested at the same time. Currently there are two types of clinical offers:

• Extended carrier screening tests. These tests are usually used by couples who are planning a pregnancy and want to know their risk of transmitting a genetic disease to their child. These tests can analyze several genes (up to 300) linked to recessive or X-chromosome-related diseases. A carrier is called an individual who has a functional copy of the gene, but another copy that contains a mutation. A carrier generally shows no symptoms of the disease. An at-risk couple consists of two carrier spouses, and they have a 25% risk of having an affected child with each pregnancy.
• “Proactive” screening tests. These tests are generally used by healthy adult individuals who want to know more about the genetic factors that affect their health. The genes tested identify diseases for which management guidelines are available (e.g., hereditary cancer, hereditary heart disease, family hypercholesterolemia, thrombophilia).

In recent years, a number of genetic tests offering information on genetic factors that have no health impact have been marketed. These tests establish your possible ancestral heritage, including features such as your aversion to coriander or your preference for certain wines. Some companies also offer analyses of genes associated with certain diseases, but these tests are often incomplete since they only analyze a few mutations (e.g., analysis of 3 mutations from a possible 2000). In addition, many of the laboratories offering the tests do not have the requisite qualifications to perform a diagnostic test. The results must therefore be confirmed by another laboratory. If you are interested in this type of test and you select an offer with an analysis of genes associated with diseases, we recommend that you first discuss this with a health professional or a genetic expert, such as a genetic counsellor.

Not all genetic tests are equal and different technologies offer sometimes different results. Main technologies include:

Caryotype: chromosome structure analysis. May identify the presence or absence of a chromosome or part of a chromosome. Cannot identify the presence of a mutation in a gene. Cannot identify excess small parts (duplication) or fewer small parts (deletion).

Microchip analysis (“CGH testing”): analyzes all 23 chromosome pairs and identifies small areas that are either in excess (duplication) or missing (deletion).

Genotyping: identifies the presence or absence of a specific gene mutation or genetic variation in a specific region of a gene. Cannot identify other genetic mutations found elsewhere in the same gene.

Gene sequencing: performs near-complete “reading” of one or more genes to identify the presence of a mutation. Only tests selected genes.

Exome sequencing: performs a near-complete “reading” of the coding parts of all the genes in an individual. May be difficult to interpret the results and clinical information (symptoms). It is often useful to compare test results with those of other family members (e.g., children and parents).

Whole genome sequencing: performs a near-complete “reading” of the entire genome (all genes, coding and non-coding parts) of an individual. Although this type of analysis covers the whole genome, the analysis is less thorough than gene sequencing (some mutations cannot be determined). In addition, the amount of information generated has several grey zones since our current knowledge of thousands of genes is still incomplete. An interpretation of these results can be difficult.

Genetic counsellors are medical genetics health professionals with a master’s degree in genetic counselling. Most genetic counsellors are certified by the Canadian Association of Genetic Counsellors or the American Board of Genetic Counseling.

The genetic counselling process is meant to help patients and their families understand and adapt to the medical and psychological implications of the genetic aspects of diseases. A genetic counselling meeting usually lasts 60 minutes and it is preferable that your genetic tests be complemented by the advice of a medical genetics specialist. An initial meeting prior to genetic testing and another one during the disclosure of results will help you understand the information you have received and use it to make informed decisions.

Genetic counselling allows you to:

• Understand your family and medical history and assess your risks
• Understand the medical aspects, transmission and possible implications of a disease
• Find out what genetic tests are available
• Decide if you want to proceed with genetic testing
• Discuss and understand your results
• Find out your health management options based on risks or your results
• Identify resources to obtain additional information and support

Genetic counselling can be done at different times:

• When planning a pregnancy to assess the risk of transmitting a genetic disease
• During pregnancy, following prenatal screening suggesting a high risk of Trisomy 21 or after ultrasound screening identifying a fetal malformation
• At birth following a positive neonatal test result for a genetic disease or if malformations or difficulties are observed
• During early childhood if the child has developmental delays
• In adulthood for genetic diseases occurring later in life (e.g., hereditary cancer, Huntington’s disease, polycystic kidney disease)